In the past, surface replacement of the acetabulum or "socket" portion of a hip prosthesis was used as a form of conservative arthroplasty in the hope of preserving femoral bone stock. This approach has essentially been abandoned, however, due to a high rate of acetabular failure resulting from several factors. For one, the large head of the femur associated with resurfacing leads to the acetabula being exposed to a large frictional torque. Secondly, acetabular implants must be thin in order to accommodate the large femoral head, and early clinical failure resulted due to mechanical loosening and volumetric wear.
In concert with bone conservation, attention to stress distribution within the femur after implantation of a prosthesis has become increasingly important, as it is now recognized that excess stresses induced by the implant may lead to further deterioration and necrosis. For example, conventional intramedullary implants, those secured in the marrow canal of the femur, may result in serious complications, in some cases only a few years after implantation, due to the unmanaged rotational and tensional forces transferred to the implant area. This situation presents a serious lack of alternatives, especially for the younger patient, as each arthroplasty procedure consumes additional bone material, thereby rendering each successive corrective surgery increasingly radical.
Such problems have led to the exploration of so-called "extramedullary" prosthetic joints such as that disclosed in U.S. Pat. No. 4,129,903 (Huggler) and in U.S. Pat. No. 4,795,473 (Grimes) both of which are incorporated herein by reference. In these approaches, the femoral shaft is cut to provide a flat surface against which a thrust plate is positioned, the plate being anchored by a tie rod extending through the proximal end of the femur and secured at a point along the lateral shaft. In both these approaches, however, the femoral cut is made at an acute angle relative to the longitudinal axis of the femoral shaft, resulting in residual play of the implant and the tie rod as the patient's weight bears down on the prosthesis during exercise of the joint. As such, forces remain which limit the lifetime of such implants.
Thus, there remains an unsatisfied need for a hip system which conserves both femoral and acetabular bone material while, at the same time, manages stresses induced through the implant, thereby reserving sufficient material to provide a host bed for primary hip arthroplasty, if subsequently required. Such a system should be particularly valuable for the younger patient.